An air gap is understood to be the distance between the brake pad and the associated friction partners on the vehicle wheel. A gap between the brake pad and the friction partner, for example a brake disk, is required so that the friction partner can move freely in the unbraked state. The air gap must not be too great in this case, however, since the proper performance of the brake could be adversely affected otherwise. On the other hand, brakes having an insufficient air gap tend to overheat. The actual air gap should therefore not fall below a lower limiting value and, on the other hand, should not exceed a predetermined upper limiting value.
The monitoring of a brake is that much more accurate, in this case, the more accurate the knowledge of the air gap is. A great deal of effort is required to determine the air gap during the travel of a motor vehicle and often succeeds only with unsatisfactory accuracy.
EP 2 520 817 A1 discloses a method for monitoring a brake for motor vehicles and an air gap detector, wherein the air gap of the brake is determined by utilizing measured position values of a brake wear sensor. The brake wear sensor is a displacement sensor which signals the wear of the brake pad. An evaluation unit detects the air gap in such a way that a warning display is activated upon detection of an increase in the air gap that is defined with respect to a predetermined setpoint air gap and, therefore, upon detection of an air gap that is too great. In the case of the known device, the brake wear sensor and the evaluation unit are designed in such a way that a present air gap is detected as the actual air gap by evaluating the signal difference provided by the brake wear sensor in the case of an actuated brake and an unactuated brake and, therefore, any deviation of the air gap from the setpoint air gap should be detectable. It has been shown, however, that a precise determination of the actual air gap is often not possible in this way. For example, displacements that are many times greater than the setpoint air gap are measured due solely to elastic expansion in the case of functional components of the brake, for example the brake caliper.